Overvoltage limiter in an aircraft electrical power generation system

ABSTRACT

A method of limiting a generator voltage in an overvoltage condition includes the steps of determining an amount of overvoltage of a generator output voltage exceeding a specified voltage and calculating a reference threshold voltage based upon the duration of overvoltage. A switch is modulated according to a voltage error between the output voltage and the reference threshold voltage. The current flow within the generator is interrupted based upon the voltage error to limit the output voltage to a desired voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure is a divisional of U.S. patent application Ser. No.13/006,576 filed Jan. 14, 2011.

BACKGROUND

This disclosure relates to a generator for a power generating system. Inparticular, the disclosure relates to an overvoltage limitingconfiguration and a method of limiting the output voltage of a generatorto a desired voltage under overvoltage conditions.

One type of aircraft electrical power generating system includes avariable frequency generator. The variable frequency generator includesa permanent magnet generator (PMG), an exciter, and a main generatormounted for rotation on a common shaft. The shaft is driven by a primemover.

A generator control unit (GCU) converts alternating current from the PMGto provide DC current to the exciter. Current from the exciter is fed tothe main generator, which produces a voltage output.

Under some fault conditions, an overvoltage condition may result, whichproduces a higher than desired output voltage from the main generator.There are many strategies for limiting or preventing overvoltageconditions, but desired overvoltage protection remains lacking. Forexample, one typical overvoltage management strategy simply trips aswitch to an open condition once a overvoltage threshold has beenreached. Another strategy delays tripping the switch depending upon theduration of the overvoltage to avoid needlessly tripping the switch fora brief overvoltage spike. In both of the above strategies, once theundesired overvoltage has occurred, the generator is de-energized andeffectively disabled, which may require the switch to be mechanicallyreset. Thus, the generator is not capable of supplying power during apersistent overvoltage condition.

SUMMARY

In one exemplary embodiment, a method of limiting a generator voltage inan overvoltage condition includes the steps of determining an amount ofovervoltage of a generator output voltage exceeding a specified voltageand calculating a reference threshold voltage based upon the duration ofovervoltage. A switch is modulated according to a voltage error betweenthe output voltage and the reference threshold voltage. The current flowwithin the generator is interrupted based upon the voltage error tolimit the output voltage to a desired voltage.

In a further embodiment of the above, the method includes using point ofregulation voltage.

In a further embodiment of the above, the method includes decreasing thereference threshold voltage as the duration of the overvoltageincreases.

In a further embodiment of the above, the switch is arranged between apermanent magnet generator and an exciter. The method includesinterrupting the current between the permanent magnet generator to theexciter.

In a further embodiment of the above, the method includes interruptingthe current along a return path from the exciter to the permanent magnetgenerator.

In a further embodiment of the above, the method includes tripping anovervoltage protection switch if at least one of a maximum overvoltageis exceeded or a duration of overvoltage condition exceeds allowablelimits.

In another exemplary embodiment, a method of limiting a generatorvoltage in an overvoltage condition includes the steps of determining anamount of overvoltage of a generator output voltage exceeding aspecified voltage. A switch is modulated to the specified voltage. Thecurrent flow within the generator is interrupted based upon the voltageair to limit the output voltage to a desired voltage.

In a further embodiment of the above, the switch is arranged between apermanent magnet generator and an exciter. The current between thepermanent magnet generator to the exciter is interrupted.

In a further embodiment of the above, the current along a return pathfrom the exciter to the permanent magnet generator is interrupted.

In a further embodiment of the above, an overvoltage protection switchis tripped if at least one of a maximum overvoltage is exceeded or aduration of overvoltage condition exceeds allowable limits.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be further understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a general schematic sectional view of a generator for a gasturbine engine.

FIG. 2 is a schematic view of an overvoltage protection arrangement fora generator.

FIG. 2A is a schematic view of an exciter field driver shown in FIG. 2with an H-bridge including a pair of MOSFETs and a pair of flybackdiodes.

FIG. 3 is a flow chart illustrating a method of limiting a generatorvoltage in an overvoltage condition.

FIG. 4 is a flow chart illustrating another method of limiting agenerator in an overvoltage condition.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates selected portions of an examplegenerator 10 driven by a prime mover 12 such as a gas turbine engine andfor generating electrical current when being driven by the prime mover12. The generator 10 may generally include a dynamoelectric portion 14,hydraulic pump 16 and a gear train 18, all contained within a commonhousing assembly 20.

The dynamoelectric portion 14 in the disclosed exemplary embodiment is a3-phase machine that includes three machines 22, 24 and 26 mounted on arotor shaft 28 along an axis of rotation A. Stator assemblies 22B, 24B,26B of the three machines are installed in the housing assembly 20, andthe three rotor assemblies 22A, 24A, 26A are installed on the rotorshaft 28. The housing assembly 20 may be closed with a drive end coverassembly or housing portion 20A, through which the rotor shaft 28extends, and an end plate 20B.

The first machine 22 includes a permanent magnet generator (PMG) with arotor assembly 22A and a stator assembly 22B. The stator assembly 22Bsupplies power for generator excitation, as well as power for othercomponents of the electrical system. The second machine 24 includes anexciter with a rotor assembly 24A and a stator assembly 24B. The exciterreceives field excitation from the PMG through a GCU 32 (GeneratorControl Unit). The output of the rotor assembly 24A is supplied to ashaft mounted diode pack 30. The diode pack 30 may be divided into sixdiode groups to provide a 3-phase full wave bridge rectification. The DCoutput of the diode pack 30 supplies the third machine 26, or maingenerator, which provides a desired output voltage from a POR 34 (Pointof Regulation).

Portions of the GCU 32 are illustrated in more detail in FIG. 2. The GCU32 includes a bridge rectifier 36 that converts 3-phase alternatingcurrent from the PMG 22 to DC power. The bridge rectifier 36 is arrangedin a circuit 38 with an exciter field driver 44, which provides power tothe exciter 24. In one example, the exciter field driver 44 includes anH-bridge having a pair of MOSFETs and a pair of fly-back diodes.

A capacitor 42 is provided in the circuit 38 to reduce the DC voltageripple from the bridge rectifier 36. An exciter controller 45 iselectrically connected between the POR 34 and the exciter field driver44. The exciter controller 45 receives point of regulation (POR) voltagefrom the POR 34 and provides a desired voltage/current command signal 47in response thereto to the exciter field driver 44 to achieve thedesired output voltage from the main generator 26. A faulty desiredvoltage/current command signal may result in improper control of theexciter field driver 44 thereby resulting in an overvoltage condition.The exciter field driver may also include a conventional overvoltageprotection switch that trips if a maximum overvoltage is exceeded or theduration of overvoltage condition exceeds allowable limits.

A switch 46, such as a MOSFET, is provided in the circuit in a returnpath 40 from the exciter field driver 44 to the bridge rectifier 36. Theswitch 46 includes open and closed conditions. Current flows through thecircuit 38 in the closed condition, and current flow is interrupted inthe open condition. An overvoltage limit controller 48 is electricallyconnected to the switch 46 and receives signals from the POR 34. Theovervoltage limit controller 48 determines an amount of overvoltageexceeding a specified voltage in an overvoltage condition by detectingall three phase voltages. In one example, the specified voltage for overvoltage condition for the main generator 26 may be 240 volts. The pointof regulation (POR) voltage from the main generator 26 may be 300 voltsfor example, corresponding to an overvoltage condition. In oneimplementation of the voltage limiter, the overvoltage threshold for thelimiter may be set at a fixed value of 280 V. The overvoltage limitcontroller 48 then modulates the switch 46 to limit the POR voltage 34to 280 V as long as the overvoltage condition exists. Thus, thegenerator 10 can continue to supply power to a component, such as anaircraft system.

Referring to FIG. 3, a method of limiting overvoltage of the generatoris generally indicated at 50. The actual POR voltage is measured, asindicated at block 51. The amount of overvoltage is compared to aspecified voltage (for example, 280V), which can be the lowest thresholdvoltage considered as overvoltage, as indicated at block 52. The switchis modulated to maintain the output voltage at the specified voltagethroughout the overvoltage condition, as indicated at block 54. As aresult, the current flow between the PMG and exciter is interrupted tolimit the voltage (block 56). If the overvoltage limit controller isunable to maintain the output voltage at or below the specified voltageand/or the output voltage exceeds a maximum overvoltage or the durationof overvoltage condition exceeds allowable limits, then the conventionalvoltage protection switch may be tripped, as indicated at block 58.

In another implementation, the overvoltage limit controller 48 uses areference voltage threshold based upon the duration of the overvoltagecondition, and modulates the switch 46 to limit the POR voltage to thereference voltage threshold; the longer the duration, the lower thereference voltage threshold will be. This varying threshold profilestarts at an upper overvoltage threshold, which is less than the maximumovervoltage, and continues to decrease as a function of time. Thisprocess occurs iteratively such that the actual POR voltage converges onthe desired voltage below the specified voltage.

A rate of change of voltage may also be used in combination with the PORvoltage to allow limiting the voltage sooner in the case of rapidlychanging POR voltages.

The overvoltage limit controller commands the switch open and closed tointerrupt current flow within the circuit 38 based upon the errorbetween the actual POR voltage, or a combination of actual POR voltageand weighted rate of change in voltage, and the reference thresholdvoltage to limit the output voltage (actual POR voltage) to the desiredvoltage. The rate of open and close of the switch is determined by thedegree of hysteresis provided at the reference voltage threshold. Thatis, operation of the switch 46 based upon the reference voltagethreshold will achieve the desired voltage at the main generator. Forexample, an output voltage of 300V may necessitate the switch to bemodulated OFF and ON for 40 ms into the overvoltage condition to limitthe output voltage to 300V and then continue to limit the output voltageto lower voltages as time progresses.

This method of limiting overvoltage of the generators is generallyindicated at 60 in FIG. 4. The actual POR voltage is measured, asindicated at block 62. The amount of overvoltage is compared to aspecified voltage, which is the lowest threshold voltage considered asovervoltage, as indicated at block 64. A reference threshold voltage iscalculated based upon the duration of overvoltage (block 66). The switchis modulated according to a voltage error between the actual POR voltageand reference threshold voltage, as indicated at block 68. As a result,the current flow between the PMG and exciter is interrupted to limit thevoltage (block 70). The steps of blocks 66, 68, 70 may be repeated toiteratively converge upon the desired voltage and achieve an outputvoltage at or below the specified voltage. If the overvoltage limitcontroller is unable to maintain the output voltage at the specifiedvoltage and/or the output voltage exceeds a maximum overvoltage or theduration of overvoltage condition exceeds allowable limits, then theconventional voltage protection switch may be tripped, as indicated atblock 72.

The switch 46 can be used and tripped when implementing the conventionalvoltage protection feature. That is, the same switch can be used forboth overvoltage limiting and overvoltage protection.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of the claims. For that reason, the following claimsshould be studied to determine their true scope and content.

What is claimed is:
 1. A method of limiting a generator voltage in anovervoltage condition comprising the steps of: a) determining an amountof overvoltage of a generator output voltage exceeding a specifiedvoltage; b) calculating a reference threshold voltage based upon theduration of overvoltage; c) modulating a switch according to a voltageerror between the output voltage and the reference threshold voltage;and d) interrupting current flow within the generator based upon thevoltage error to limit the output voltage to a desired voltage.
 2. Themethod according to claim 1, wherein step a) includes using point ofregulation voltage.
 3. The method according to claim 1, wherein step b)includes decreasing the reference threshold voltage as the duration ofthe overvoltage increases.
 4. The method according to claim 1, whereinthe switch is arranged between a permanent magnet generator and anexciter, and step d) includes interrupting the current between thepermanent magnet generator to the exciter.
 5. The method according toclaim 4, wherein step d) includes interrupting the current along areturn path from the exciter to the permanent magnet generator.
 6. Themethod according to claim 1, comprising step e) tripping an overvoltageprotection switch if at least one of a maximum overvoltage is exceededor a duration of overvoltage condition exceeds allowable limits.
 7. Amethod of limiting a generator voltage in an overvoltage conditioncomprising the steps of: a) determining an amount of overvoltage of agenerator output voltage exceeding a specified voltage; b) modulating aswitch to the specified voltage; and c) interrupting current flow withinthe generator based upon the voltage air to limit the output voltage toa desired voltage.
 8. The method according to claim 7, wherein theswitch is arranged between a permanent magnet generator and an exciter,and step c) includes interrupting the current between the permanentmagnet generator to the exciter.
 9. The method according to claim 8,wherein step c) includes interrupting the current along a return pathfrom the exciter to the permanent magnet generator.
 10. The methodaccording to claim 7, comprising step d) tripping an overvoltageprotection switch if at least one of a maximum overvoltage is exceededor a duration of overvoltage condition exceeds allowable limits.